In the production of blown film, the film-making apparatus comprises an extruder for extruding a thermoplastified synthetic resin tube which is expanded by blowing and is passed through a calibrating basket which serves to define the expanded diameter of the blown tube and thus determines the thickness of the film.
The calibrating basket is provided with guide stirrups which guide the synthetic resin tube and have foil guide rollers whose outer peripheries contact the film of the tube.
In such a calibrating basket, around the periphery of the calibrating basket and thus over the periphery of the synthetic resin tube guided through it, there are distributed a multiplicity of such guide stirrups. These guide stirrups are distributed in the longitudinal direction of the calibrating basket and, therefore, the direction of travel of the tube. The guide stirrups are provided with guide rods on which a multiplicity of film guide rollers can be provided next to on another, i.e. in neighboring relationship. The guide rods of a guide stirrup can be bent to match the shape of the synthetic resin tube.
In the past, the rollers have operated in accordance with the slide-bearing principle, i.e. were free from rolling surfaces and had relatively movable surfaces that slid on one another. These slide surfaces, could be made from a low friction material like polytetrafluoroethylene. In a special case, the guide rollers were hollow cylindrical bodies made from Teflon and were slipped onto the guide rods. Slide bearing rollers rotate around the guide rods or rest thereon generally in such manner that there are two points of contact between the slide bearing roller and the rod. A substantial amount of friction heat is generated with these rollers and sliding surfaces may even partially melt because of the friction heat which is generated. This melting of the guide rollers can result in fusion of them to the guide rods so that the guide rollers no longer are rotatable or can give rise to irregular rotation.
Immobilization of such a roller can result in the undesirable marking of the film with contact stripes. When some rollers rotate and others do not, the film may be pinched between adjacent rollers and that has a detrimental effect on film quality or increases the reject rate.
Conventional film guide rollers of the type described also have problems when the foil is somewhat tacky since then the friction between the foil rollers may be excessive and the rollers may develop rotational oscillation which can also result in film marking or detrimental calibration.